The present invention relates to multicomponent fibers, as well as nonwoven fabrics and fabric laminates which comprise the multicomponent fibers. More particularly, the invention relates to multicomponent fibers which include at least one polymer domain formed of a select combination of polymers, as well as nonwoven fabrics and laminates having improved fabric properties and processing characteristics.
Nonwoven fabrics produced from spun polymer materials are used in a variety of different applications. Among other uses, such nonwoven fabrics are employed as the cover sheet for disposable diapers or sanitary products. There is considerable interest in making disposable diapers more comfortable and better fitting to the baby. An important part of the diaper comfort is the softness or hardness of the nonwovens used to make the diaper, including the diaper topsheet, barrier leg cuffs, and in some advanced designs, the fabric laminated to the backsheet film. In addition, in some diaper designs, a high degree of fabric elongation is needed to cooperate with elastic components for achieving a soft comfortable fit.
One approach to improved diaper topsheet softness is to use linear low density polyethylene (LLDPE) as the resin instead of polypropylene for producing spunbonded diaper nonwoven fabrics. For example, Fowells U.S. Pat. No. 4,644,045 describes spunbonded nonwoven fabrics having excellent softness properties produced from linear low density polyethylene. However, the above-described softness of LLDPE spunbonded fabric has never been widely utilized because of the difficulty in achieving acceptable abrasion resistance in such products. The bonding of LLDPE filaments into a spunbonded web with acceptable abrasion resistance has proven to be very difficult. Acceptable fiber tie down is observed at a temperature just below the point that the filaments begin to melt and stick to the calender. This very narrow bonding window has made the production of LLDPE spunbond fabrics with acceptable abrasion resistance very difficult. Thus, the softness advantage offered by LLDPE spunbonded fabrics has not been successfully captured in the marketplace.
Conventional polypropylene, which has been widely used in producing nonwoven fabrics, provides adequate fuzz and abrasion resistance properties in the unstretched condition, but the elongation properties are unacceptable and therefore the fibers and/or fabrics fracture.
In Sabee, U.S. Pat. Nos. 4,153,664 and 4,223,063, it is disclosed that the softness and drapeability of composite nonwoven fabrics, formed for example from a meltblown or a spunbonded nonwoven fabric, can be improved by drawing or stretching the fabric. More particularly, according to Sabee, the composite nonwoven fabrics are processed by differentially drawing or stretching the web to form a quilted pattern of drawn and undrawn areas, providing a product with enhanced softness, texture and drapeability. However, while the stretching may improve some fabric physical properties, it can adversely affect other important properties, such as abrasion resistance, for example, leaving the fabric with an unsightly fuzzed surface. In addition, Sabee teaches the use of undrawn or underdrawn filaments in the use of this application. Undrawn or underdrawn filaments are typically higher in denier and therefore the fabrics tend to be stiff. In addition to softness, often the performance requirements of the product demand a composite nonwoven fabric having elasticity. In certain disposable diaper designs, for example, it is desired to impart elastic properties to the waist and/or to the leg cuff areas. One approach which has been taken to providing such elastic properties in a composite nonwoven fabric involves forming and stretching an elastic web, then bonding a gatherable web to the elastic web, and relaxing the composite. An obvious limitation of this approach is having to form the composite in the tensioned state. This requires additional equipment and control systems. Examples of this process are Mormon, U.S. Pat. No. 4,657,802, where it is disclosed that a composite nonwoven elastic is made by first stretching an elastic web, forming a fibrous nonwoven gatherable web onto the stretched elastic nonwoven, joining the two together to form a composite structure, then allowing the composite to relax. In Collier, et al., U.S. Pat. No. 5,169,706, it is disclosed that a composite elastic material having a low stress relaxation is formed between an elastic sheet and a gatherable layer. In Daponte, U.S. Pat. No. 4,863,779, a composite is disclosed which involves first tensioning the elastic elastic web to elongate it, bonding at least one gatherable web to the elastic web, and relaxing the composite immediately after bonding, so that the gatherable web is gathered between the bond points.
Another approach to imparting elastic properties to a composite nonwoven fabric is with a so-called xe2x80x9czero-strainxe2x80x9d stretchable laminate. A xe2x80x9czero-strainxe2x80x9d stretchable laminate refers to a fabric in which at least two layers of material, one elastic, the other substantially inelastic, are secured to one another along their coextensive surfaces while in a substantially untensioned state. The fabric is subsequently subjected to mechanical stretching. The inelastic layer typically fractures or extends, thus permanently elongating the inelastic layer and producing a composite fabric with elastic properties. This lamination and stretching process is advantageous in that utilizing elastic in an unstretched condition is easier and less expensive than stretched elastic used in traditional processing operations. However, one problem which has existed with presently available xe2x80x9czero-strainxe2x80x9d stretchable laminates is surface abrasion. The mechanical stretching either fractures or disrupts the fibers within the substantially inelastic component of the xe2x80x9czero-strainxe2x80x9d laminate, and as a result, the fibers detach and are susceptible to linting and pilling. In addition, such fracturing or detachment causes a noticeable loss in fabric strength.
There have been attempts to address the aforementioned problems of fiber tie down and fabric abrasion resistance. For example, attempts have been made to make the nonwoven fabric component of the composite with high elongation properties. Conventional polypropylene, as noted above, which has been widely used in producing nonwoven fabrics, provides adequate fuzz and abrasion resistance properties in the unstretched condition, but the elongation properties are unacceptable and therefore the fibers and/or fabrics fracture. Nonwoven webs formed from linear low density polyethylene (LLDPE) have been shown to have high elongation properties and also to possess excellent hand, softness and drape properties. However, as also noted above, such fabrics have not found wide commercial acceptance, since they fail to provide acceptable abrasion resistance.
The present invention overcomes these disadvantages and limitations and provides multicomponent fibers and nonwoven fabrics formed of the same having a superior combination of extensibility, tensile properties and abrasion resistance. The multicomponent fibers of the invention include at least two polymer components arranged in structured domains. At least one of the polymer components is formed of a select blend of specific grades of polyethylene and polypropylene which give improved fabric performance not heretofore recognized or described, such as high abrasion resistance, good tensile properties, excellent softness and the like. Furthermore, these blends have excellent melt spinning and processing properties which permit efficiently producing nonwoven fabrics at high productivity levels.
The present invention is based upon the discovery that blending a relatively small proportion of polypropylene of a select class with the polyethylene imparts greatly increased abrasion resistance to a nonwoven fabric formed from the polymer blend, without significant adverse effect on the fabric softness properties. It is believed that the polyethylene and the polypropylene form distinct phases in the filaments. The lower-melting polyethylene is present as a dominant continuous phase and the higher-melting polypropylene is dispersed in the dominant polyethylene phase.
The multicomponent fibers can be continuous filaments, staple fibers, or meltblown fibers. Advantageously, at least a portion of the outer surface of the multicomponent fiber includes a polymer domain formed of the polymer blend to impart desired elongation, softness and abrasion resistance properties to the fibers and in turn to fabrics formed of the fibers. In a preferred embodiment, the fibers are bicomponent fibers with the polymer components arranged in a sheath-core structured domain. In this aspect of the invention, the sheath is formed of the polymer blend to impart the desired properties to the fibers and in turn to fabric produced using the same.
The physical and rheological behavior of these blends is part of a phenomenon observed by applicants wherein a small amount of a higher modulus polymer reinforces a softer, lower-modulus polymer and gives the blend better spinning, bonding and strength characteristics than the individual constituents. The lower melting, relatively low modulus polyethylene provides desirable properties such as softness, elongation and drape; while the higher-melting, higher modulus polypropylene phase imparts one or more of the following properties to the dominant phase: improved ability to bond the web; improved filament tie-down (reduces fuzz); improved web properties- tensiles, and/or elongation and/or toughness; rheological characteristics which improve spinning performance and/or web formation (filament distribution).
According to one advantageous aspect of the present invention, the lower-melting continuous phase comprises a linear low density polyethylene polymer of a melt index of greater than 10 (ASTM D1238-89, 190xc2x0 C.) and a density of less than 0.945 g/cc (ASTMD-792). At least one higher-melting noncontinuous phase comprises a polypropylene polymer with melt flow rate of greater than 20 g/10 min (ASTM D1238-89, 230xc2x0 C.). In one of the preferred embodiments of the invention, the lower-melting continuous phase of the blend comprises a linear low density polyethylene having a density of 0.90-0.945 g/cc and a melt index of greater than 25 g/10 minutes. In another preferred embodiment, the lower-melting polymer phase comprises linear low density polyethylene as described above and the higher-melting polymer phase comprises an isotactic polypropylene with a melt flow rate greater than 30 g/10 minutes. In another embodiment of the invention, the lower-melting polymer phase comprises linear low density polyethylene with a melt index of 27 and the higher-melting polymer phase comprises an isotactic polypropylene with a melt flow rate of 35 g/10 minutes.
According to another aspect of the present invention, the lower-melting dominant continuous phase is blended with a higher-melting noncontinuous phase of propylene co- and/or ter-polymers. When propylene co- and/or ter-polymers are used as the higher-melting noncontinuous phase, the lower melting continuous phase may be comprised of one or more polyethylenes selected from the group consisting of low density polyethylene, high pressure long chain branched polyethylene, linear low density polyethylene, high density polyethylene and copolymers thereof.
The multicomponent fibers of the invention are highly elongatable and are useful in the production of coherent extensible nonwoven fabrics having desirable yet contradictory properties. Specifically, the fibers of the invention can be formed into fabric exhibiting good softness, abrasion resistance and elongation. According to one embodiment of the present invention, the coherent extensible nonwoven web is a thermally bonded spunbond nonwoven web of randomly arranged substantially multicomponent continuous filaments, in which at least one polymer domain is formed of multiple polymers. According to another embodiment of the invention, the coherent extensible nonwoven web is a thermally bonded carded web of multicomponent staple fibers. The coherent extensible nonwoven web may contain, in addition to the multicomponent fibers, additional fibrous components, such as meltblown microfibers. The fabrics of the present invention can have a Taber surface abrasion value (rubber wheel) of greater than 10 cycles and an elongation at peak load in at least one of the machine direction or the cross-machine direction of at least 70%.
In accordance with another embodiment of the invention, the nonwoven fabric may include one or more additional layers or components laminated thereto. Exemplary additional layers include continuous or perforated polymer films, films or webs of an elastic polymer, spunbonded nonwoven webs, extensible scrims or nets, an array of extensible or elastic strands, a web of meltblown microfibers, a web of staple fibers, and the like. Where an elastic web or film is used, the composite can be stretch activated by elongation, which causes permanent elongation and stretching of the coherent extensible web of multicomponent fibers, and the resulting composite fabric exhibits elastic properties. Where an extensible nonelastic layer is used, such as polyolefin film for example, the composite can be stretch activated by elongation, for example to at least 20% of its original unstretched length, producing a composite having excellent softness and drape.
The resultant composite fabrics can have a cloth like hand and good cover characteristics suitable for use in disposable absorbent garments, such as diapers, incontinence pads, sanitary napkins and the like. The composite fabrics are particularly useful as components of disposable diapers, such as in leg barrier cuffs, side panels, backsheet, topsheet, and the like.